1. Field of the Invention
The present invention relates generally to a liquid discharging or ejecting recording head of the type in which a heat energy is applied to a recording liquid so that the recording liquid is converted into liquid droplets which in turn are discharged or ejected from the recording head in order to record data and more particularly to a liquid discharging or ejecting head in which a temperature of such a head is compensated.
2. Description of the Prior Art
Liquid discharging or ejecting recording methods such as a so-called ink jet recording method have recently been attracting an increasing interest because noise produced at the time of recording is almost negligible; a high speed recording is possible; and the liquid discharging or ejecting recording is made on plain paper without requiring any special fixing process.
Of these methods, the liquid discharging recording method disclosed in Japanese Patent Application Laying-open No. 54-51837 and German Patent Application Laying-open (DOLS) No. 2843064 has a feature different from the other methods especially in that a thermal energy is applied to recording liquid so as to obtain an energy for jetting liquid droplets.
More particularly, according to this method, when recording liquid is actuated by a thermal energy, the recording liquid changes its state, involving rapid expansion of a volume of the recording liquid. As a result of this change of state, the recording liquid is ejected from an orifice such as a liquid discharging opening positioned at the end of the recording head by a force based upon the change of the state to form a frying droplet. The frying droplet lands on a recording medium such as recording paper, so that recording is made on the paper.
The liquid discharging recording method of the type disclosed in DOLS No. 2843064 is advantageously applied to a so-called drop-on demand recording method. Furthermore, according to this method, a recording head with a high degree of multi-orifice in the form of a full line can be easily constructed. Thus, the liquid recording head has a feature in that a high quality image with a high degree of resolution can be obtained at a high speed.
A recording head used in a recording apparatus constructed according to the above-mentioned method includes, in general, a liquid discharging portion having a plurality of orifices each of which ejects recording liquid to form a frying droplet and a plurality of recording liquid passages which partially have a thermal energy application portion for applying a thermal energy for discharging a droplet to the recording liquid; and means for generating the thermal energy.
So far, a liquid discharging recording head is constructed in the manner described above. Such a head, however, has various problems to be solved as will be described below.
Firstly, some problems are caused by temperature characteristics of the liquid discharging recording head. Concerning a relation of a size of a recording dot formed by a recording liquid droplet landed on recording paper, i.e., a dot diameter with a temperature of the recording head, the dot diameter is closely dependent upon a temperature of the recording head. The reason is that in accordance with variations in temperature of the recording head, an initial bubble forming force required for forming a recording liquid droplet varies over a wide range. Especially when the temperature is low, the initial bubble forming force applied to the recording liquid is small, so that a frying recording liquid droplet cannot be formed in a stable manner. As a result, it is impossible to obtain a high quality dot image.
In order to overcome this problem, so far an external heating type heater such as a positive characteristic thermistor is used to heat the entire recording head from the exterior of the head. According to this method, however, the whole recording head is heated so that there are problems that power consumption is higher and that the response speed of temperature rise response is slow.
Furthermore, a liquid discharging head which utilizes thermal energy involves self-heating in principle and the recording liquid flows over a substrate so that the substrate is cooled. As a result, a temperature distribution of the head is complicated. As a consequence, in the case of a liquid discharging recording head of the type having a plurality of nozzles (a multi-orifice liquid discharging recording head), it is imperative to uniform the temperature distribution and to improve the characteristics of the recording head at a low temperature in order to obtain a high quality image.
With the above in view, it has been proposed to arrange integrally heating means such as a compensating heater for temperature compensation on a heater board as a substrate having thereon thermal energy generating means for discharging the recording liquid, for example, a discharging heater, thereby increasing a thermal transmission efficiency and accordingly decreasing electric power consumption and enhancing the response speed.
When the discharging heater and the compensating heater are disposed on the heater board in closely spaced relationship in the manner described above, the thermal energy transmission efficiency is improved so that a temperature rise time required for a temperature compensation starting from low temperature is greatly different from a temperature rise time required for a temperature compensation starting from room temperature. In addition, a temperature rise time at a starting time that an electric power source is turned on to start using the recording head is different from a temperature rise time at a waiting time after a series of recording operations are terminated. In other words, a time period required for temperature compensation varies in response to an environmental temperature and an operation condition of the printer. It follows, therefore, that if a heating operation is carried out uniformly regardless of the starting time or the waiting time, for instance, the waiting time for the recording processing is excessively elongated due to the uniform heating operation, so that there is the possibility that printing errors occur.
In order to solve the above-mentioned problem, it can be proposed to provide a temperature sensor and means for varying an electrical energy to be applied to the compensating heater in response to a detection signal from the temperature sensor. This solution, however, involves a problem that the cost of the recording head is expensive. It can also be proposed to apply an over power to the compensating heater so that a waiting time can be reduced under any condition of the recording head. There arises, however, another problem from the standpoint of durability of the compensating heater and its energy consumption.